The development of an effective HIV/AIDS vaccine remains a high international health priority as the most cost-effective means to stem the AIDS pandemic. At this point in time, there are very few general HIV/AIDS vaccine strategies that remain viable for clinical development as a prophylactic HAV/AIDS vaccine - that is, vaccine platforms that have shown promising efficacy in preclinical studies and not been proved ineffective in human clinical trials. Among these is the approach developed by our group in which persistent Cytomegalovirus (CMV)-derived vectors are used to elicit high-frequency, indefinitely persistent HIV/SIV- specific effector-memory T cell responses. In the preclinical rhesus macaque (RM) - SIV model, we have demonstrated that after mucosal SIV challenge >50% of monkeys vaccinated with strain 68-1 RhCMV/SIVgag, /SIVpol, /SIVenv, /SIVrevtatnef vectors show complete, durable protection and eventual clearance by virologic and immunologic criteria. Protection is likely determined by the ability of effector T cells to intercept a nascent HIV/SIV infection immediately upon acquisition (with no response delay due to requirement for anamnestic expansion and effector differentiation). If this assumption is correct one could speculate that a greater efficacy could be achieved if the earliest HIV/SIV immunogens could be targeted within hours after initial infection. Recently a novel set of 5' leader sequence-encoded HIV and SIV polypeptides was discovered, and these polypeptides are expressed very early in HIV/SIV-infected cells and were highly immunogenic. Therefore, the 5' leader polypeptides (5'-LP) are an extremely attractive vaccine target considering that every spliced and unspliced HIV/SIV mRNA contains the 5' leader sequence, and thus polypeptides encoded from this region will be ubiquitously expressed rapidly and early following infection. Thus, the major objective of the research proposed here will be to determine whether RhCMV vectors expressing SIV 5' leader polypeptides can protect rhesus macaques from mucosal challenge with highly pathogenic SIVmac239, and if these vectors in combination with vectors expressing conventional SIV open reading frames, can enhance the overall protective efficacy of the RhCMV-vectored vaccine. These objectives will be accomplished by experimentally addressing the following Specific Aims: (i) To characterize the immunogenicity of strain 68-1 RhCMV/5'-LP vectors in rhesus macaques and to use this immunogenicity, in combination with biochemical analysis, to define the 5'-LP epitopes expressed by SIV-infected CD4+ T cells; (ii) To determine the efficacy of strain 68-1 RhCMV/5'-LP vector vaccination against limiting-dose, intra-rectal SIV challenge; and if Go/No-Go criteria are met, (iii) To determine the immunogenicity and efficacy of a combination vaccine containing both strain 68-1 RhCMV/5'- LP vectors and strain 68-1 RhCMV vectors expressing Gag, Rev/Nef/Tat, Pol, Env.